1. Field of the Invention
The present invention relates to a superluminescent diode, and more particularly, to a very broad band superluminescent diode obtained by growing in series two or more active layers along the length of an active waveguide and the application of such a superluminescent diode as an optical amplifier.
2. Description of the Prior Art
A superluminescent diode (SLD) is a diode in which there is stimulated emission with amplification, but insufficient feedback for oscillations to build up to achieve lasing action. SLDs are made by the same processes as laser diodes, and like laser diodes, can be fabricated to operate at various wavelengths such as 835 nm, 960 nm, 1300 nm, and 1550 nm. SLDs are used as light sources in applications such as fiber optic gyroscopes, fiber optic sensors, optical coherence tomography (OCT), and communications.
SLDs offer higher power output as compared to conventional light emitting diodes (LEDs), and offer broader spectral range (lower coherence) than lasers. The broadband width is similar to but not as wide as an LED, but the output power is as high as that of a semiconductor laser. Thus, SLDs can emit low coherent light at high output power with good directionality. In designing and producing high power SLDs, it is important to prevent laser oscillation in order for the output light to be the amplified spontaneous emission spectrum.
The full width half maximum (FWHM) bandwidth .DELTA..lambda. of SLDs is on the order of about 2.0 to about 2.5% of the peak emission wavelength of the active layers. Thus, a 1550 nm SLD would have a FWHM output spectrum of about 35 nm. Wavelength division multiplex (WDM) communication systems, in which several laser wavelengths are used as carriers in optical communications systems, require much larger bandwidths, typically on the order of 100 nm. The same is true with OCT systems that are under development for medical imaging and diagnostics instruments which require light sources with short coherence length for high depth resolution (resolving features over depths of a few microns). The coherence length is inversely proportional to the bandwidth; hence, the broader the SLD bandwidth, the shorter the coherence length, and the higher the resolution of OCT systems. It is thus desired to develop very large bandwidth SLDs.
A semiconductor laser diode structure, which is the basis of an SLD, generally consists of one or more active layers sandwiched between n-doped and p-doped cladding layers, all deposited on a single crystal substrate. In modern devices, the layers are very thin layers called quantum well (QW) layers, and they are typically deposited by MOCVD (metal organic chemical vapor deposition). The emission wavelength is determined by the thickness, composition, and strain of the active layer. A highly doped capping layer, such as a p-doped layer, is deposited over the device to facilitate contact with electrodes supplying the drive current. The bottom of the substrate is also processed with a conductor, such as n-doped metal, in order to facilitate contact and to enable current injection. An optical waveguide is created in the structure by evaporating a dielectric on the p-side in which a narrow stripe is removed for metal contact by means of evaporated metal layers on top of the dielectric. The device is completed by cleaving it into small bars or chips.
FIG. 1(a) shows a prior art ridge waveguide laser structure. An n type cladding layer 3 is deposited on a substrate 2. An undoped active layer 4 is deposited on the n type cladding layer 3. A p type cladding layer 5 is deposited on the undoped active layer 4. It is preferable that the refractive index of the undoped active layer 4 is greater than the refractive index of the two cladding layers 3, 5. The cladding layer 5 typically comprises a first cladding layer and a second cladding layer separated by an etch stop layer (not shown).
A capping layer 6 is deposited on the p type cladding layer 5. After the capping layer 6 is deposited, photolithography and etching is performed to define the waveguide as a ridge 8 with channels 9 on the sides. The channels are patterned and the capping layer 6 and the cladding layer 5 are etched down to the etch stop layer. Thus, in the channels, a small portion of the cladding layer 5 overlies the undoped active layer 4. An electrical contact 1 is then deposited to overlie the surface of the substrate 2 opposite the n type cladding layer 3. The electrical contact 1 preferably comprises at least one of germanium, gold, and nickel. A dielectric is then deposited over the entire top surface of the structure. Using photolithography and etching, a stripe is opened over the ridge 8, and a metal is deposited therein on the capping layer 6 as a second electrical contact 7 in the stripe region. Thus, current will flow only in the ridge region. The second contact comprises at least one of titanium, platinum, and gold.
A laser is made by processing the contact stripe so that it is perpendicular to the cleaved facets a and b in order to form a cavity using the facet reflections R1 and R2, as shown in the ASE (amplified spontaneous emission) region in FIG. 1(b). Because of the cavity action, the output spectrum of the laser is very narrow, as shown in FIG. 1(c). An SLD is made by fabricating the waveguide stripe of FIG. 1(b) at an angle .theta. with respect to the facets a and b to avoid facet reflection, as shown in FIGS. 2(a) and 2(b). The output spectrum of the SLD is broad, as shown in FIG. 2(c).
It is difficult to broaden the SLD output spectrum beyond the natural width shown in FIG. 2(c). One structure that has been proposed to broaden the spectrum is to stack or stagger QW layers of different thickness and composition within the active layer structure, each layer having a slightly different emission wavelength. An example of such an SLD is the layer stack shown in FIG. 3(a) and comprises three layers of materials or three groups of QW layers having respective center emission wavelengths .lambda.1, .lambda.2, and .lambda.3 and FWHM of .DELTA..lambda.1, .DELTA..lambda.2, and .DELTA..lambda.3, respectively. The three layers or groups of layers of materials in the stack can be separated by buffering materials. This stack approach has been tried, but it does not produce the desired results because the material having the longer wavelength absorbs the light emitted by the material(s) having the shorter wavelength(s). For example, in FIG. 3(a), .lambda.1&gt;.lambda.2&gt;.lambda.3, then both .lambda.2 and .lambda.3 are absorbed by the .lambda.1 material to emit more light centered at .lambda.1, with the result that the output spectrum .DELTA..lambda. is essentially the same as that of the .lambda.1 material, as shown in FIG. 3(b).
Although the art of superluminescent diodes is well developed, there remain some problems inherent in this technology, particularly with respect to the bandwidth of the output spectrum. Therefore, a need exists for an SLD having a broadened output spectrum and that overcomes the drawbacks of the prior art. The present invention has been developed for this purpose.